I have drain flies and we can't find the source. We have taped up all the drains, sinks and vents. Removed the toilet and we still can't find the sources - where they are coming from. We live in a 50 year old house. The plumber said we should do a peppermint test. Anther plumber said that is worthless. We have had these pesky flies for 2 years and I can't take it and don't even know who can get rid of these.
Are you sure they are drain flies?
Take a look at this page from our website about them and how to control them: